Hi Mike, There is a penalty to be paid if diodes are used to clamp the level of strong signals in a receiver's signal path, and that is the generation of intermodulation products. In a "no-split" pile-up situation where two or more closely spaced strong signals from other callers are in the passband, but not zero beat with the DX, chances are that an intermodulation product will land on top of the DX's signal if he returns to somebody before the others stop calling. You may ask how a weak DX station could be heard at all in this situation even if intermodulation products did not exist, to which my reply must be that it is possible.
>From reading the posts, my impression is that this case of in-passband strong signal overload could be viewed in the same fashion as a classic example of 3rd Order Dynamic Range droop or "bottoming". There is a similarity in the causes even though only a single signal is involved in the overload case. Among the usual causes for the dynamic range droop problem are (1) too much gain in the front end before the roofer to overcome the insertion loss of the roofer, and to maintain a useful overall noise figure, or (2) The Input IP3 of the roofer itself is too low, or (3) the 3rd Order Dynamic Range of the IF cascade and what follows is inadequate, or (4) a combination of these three. All of these are relevant when considering overload. According to my back of envelope calculations, the 3rd Order Dynamic Range of the K3's 8.215 MHz IF amplifier and the second mixer is in the region of 80db before the onset of hardware AGC, the second mixer being the dominant element. This calculation does not include the stages after the second mixer nor the roofer nor LO phase noise, so I believe that the actual in-passband dynamic range looking into the roofer would be less than 80db. I can only speculate on the effect that the application of hardware AGC might have on the dynamic range of this part of the receiver. What this means of course is that while the overall 3rd Order Dynamic Range of the K3 is very good in an enviroment where strong unwanted signals are outside of the roofer's passband, one should expect some decrease within the roofer's passband, likewise possible overload. The cure could be difficult to implement. 73, Geoff GM4ESD Mike Harris" <[email protected]> wrote on Monday, May 04, 2009 at 7:20 PM > Just been looking through my archives. I remembered that there was a > simple mod to the K2 to overcome audio distortion in an extreme overload > environment, I installed it on my K2 and it was totally immune to > co-channel audio distortion when my very near neighbour VP8LP transmitted: > > "K2 - Improved Handling of Extremely strong (and nearby) Signals". > > Maybe the K3 architecture offers similar opportunities. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
